Android smartphone and tablet users: Beware attackers who come gunning for a Google-issued authentication token that allows your device to automatically log into Google Apps, Gmail, Google Drive, or any other Google service.

That warning comes by way of Tripwire security researcher Craig Young's Saturday presentation at the Def Con information security conference in Las Vegas, in which Young detailed how a "weblogin" token issued by Google to Android users -- each token is unique -- could be intercepted and abused by an attacker.

A weblogin token allows an Android user to log into a desired Google service in lieu of having to enter a password. Accordingly, any attacker able to obtain a user's token could access any Google service that the Android device is configured to use. Furthermore, any attacker who gained root access -- using malware -- or physical access to an Android device would likewise be able to retrieve the token and gain carte-blanche access to any Google service authorized for use on the device, including Google Apps, Gmail and Google Drive.

"Android trades security for convenience," said Young, noting in a blog post that he discovered the vulnerabilities after reviewing the Android API. That led him to quickly prototype several iterations of Android applications designed to mislead the user and gain complete account access without passwords or two-step verification codes.

In addition, Young said, "I found several attack vectors which make it possible for an adversary targeting a single Android device to compromise an entire organization." That could occur if an intercepted Android token was issued to a security administrator who had super-user permissions -- for example, to add or delete users from a business's Google Apps domain or to alter their access privileges.

To test the attack vectors, Young crafted a proof-of-concept Stock Viewer app that he uploaded to Google Play in March. If installed, the app intercepted an Android user's weblogin token. Lest anyone criticize Young for putting people at risk, the security researcher said his app carried a disclaimer that it was for testing purposes only and would "completely [compromise] your privacy." To further dissuade anyone from downloading it, he also slapped the app with a $150 price tag.

Interestingly, Google approved the app for sale, meaning that Bouncer, its automated malware detection service, failed to detect the built-in token-grabbing functionality. Similarly, mobile antivirus software from Avast, Lookout, Norton, Sophos and Trend Micro didn't flag the software as posing a risk, or apparently detect that the software had access to the weblogin token.

Google did, however, drop the app from Google Play about a month after it was uploaded. Furthermore, prior to his Def Con talk, Young said that Google's Apps Verify feature -- which is now automatically installed on all Android devices (version 2.3 or above) that want to access Google Play -- had begun warning that the app was malicious before users install it. But Young said that warning disappeared if the app was renamed.

A Google spokesman didn't immediately respond to an emailed request for comment on Young's research. But in advance of the talk, Google had already addressed one attack vector highlighted by Young, which would have allowed someone in possession of a token to reset an account password if two-step verification wasn't enabled. In addition, Google now blocks anyone who logs into a Google account using a weblogin token from adding a user to a Google Apps domain or from obtaining a dump of all data stored by the account.

"Google is closing more weblogin Apps attacks for my #DEFCON talk -- a nice start but still a long long way to go," Young tweeted in advance of his presentation. Notably, Young has called on Google to give Google Apps administrators the ability to block all automatic access via weblogin tokens.

What can Android users do now to mitigate the vulnerability? Young recommended they never use an admin account on Android and regard all token requests made by apps with suspicion. He also recommended that users stick with trusted app stores and vendors -- which is long-standing security advice -- and run antivirus to detect root exploits, which could be used to facilitate weblogin token stealing.

Published: 2015-03-31The build_index_from_tree function in index.py in Dulwich before 0.9.9 allows remote attackers to execute arbitrary code via a commit with a directory path starting with .git/, which is not properly handled when checking out a working tree.